1. Field of the Invention
This invention relates to circuits used to generate reference currents and reference voltages on a semiconductor device and, more particularly, to low power supply voltage circuits capable of generating reference currents and reference voltages on a semiconductor device with high accuracy and reduced sensitivity to power supply noise.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Bandgap reference circuits are known for generating reference voltages, which exhibit little variation across defined ranges of temperatures, process corners and power supply voltages. FIG. 1 shows an exemplary block diagram for a Bandgap reference circuit. Circuit 100 in FIG. 1 generates a reference voltage VREF as a weighted sum of two voltages: V1, having a positive temperature coefficient (TCPOSV), and V2, having a negative temperature coefficient (TCNEGV). The reference voltage may, therefore, be expressed as:VREF=α1*V1=α2*V2  (1)whereTCPOSV=d(V1)/dT>0, and  (2)TCNEGV=d(V2)/dT<0.  (3)In equations (2) and (3) above, V1 is proportional to absolute temperature (PTAT), V2 is linearly decreasing with absolute temperature (CTAT, complementary with absolute temperature) and α1, α2 are non-dimensional coefficients.
As shown in the graph of FIG. 2, Bandgap circuit 100 may be used to provide a relatively constant reference voltage VREF across a defined range of temperatures if the coefficients α1, α2 are chosen such that there is a temperature T0 for which:d(VREF)/dT=α1*TCPOSV+α2*TCNEGV=0 at T=T0  (4)where T is the absolute temperature (K) and T−x<T0<T+x. T−x, T+x define the range of temperatures for which voltage generation circuit 100 is specified to work. Bandgap reference circuit 100 may alternately be referred to as a “Voltage output Bandgap circuit”.
Alternately, a nominally constant reference voltage VREF across a specified range of temperatures may be generated by creating a reference current and then passing it through a resistor. In one example, circuit 300 (FIG. 3a) is used to generate a reference current IOUT as a weighted sum of two currents: I1, having a positive temperature coefficient (TCPOS1), and I2, having a negative temperature coefficient (TCNEGI). In other words, I1 is PTAT and I2 is CTAT. The reference current value may, therefore, be expressed as:IOUT=β1*I1+β2*I2  (5)where β1 and β2 are non-dimensional coefficient values chosen to minimize temperature-dependent variations in the reference current across the range of temperatures considered.
The reference voltage VREF may be generated by passing the reference current IOUT generated by circuit 300 through a resistor of value R such that:VREF=R*IOUT  (6)In this manner, the generated reference voltage VREF demonstrates a relatively small variation (i.e., a small ΔVREF, as shown in FIG. 2) over the range of temperatures considered, if temperature-dependent variations in IOUT are minimized. It is to be noted that the temperature coefficient of the resistor R also plays an important role in defining the variation of VREF with temperature. Additional sources of error associated with circuit 300 will be discussed in more detail below. The small variation of VREF with temperature is implemented by selecting appropriate values for the coefficients β1 and β2 in equation (5) such that the generated reference voltage VREF has the property:d(VREF)/dT=0 at T=T0  (7)where T is the absolute temperature (K) and T−x<T0<T+x. T−x, T+x define the range of temperatures for which current generation circuit 300 is specified to work. Circuit 300 may alternately be referred to as a “Current output Bandgap circuit”.
In some cases, the negative temperature coefficient current, I2 (CTAT), can be generated in circuit 300 by developing a forward voltage (VD1) of a p-n junction diode across a resistor (R1), such that:I2=VD1/R  (8)Alternately, I2 can be generated by developing a base-emitter voltage (VBE) of a bipolar junction transistor (BJT) across a resistor (R1) when the BJT is biased in normal active mode. As used herein, a “normal active mode of operation” for a BJT refers to the case when the base-emitter junction of the BJT is forward biased and the base collector junction of the BJT is reverse biased.
In Current output Bandgap circuit 300, the positive temperature coefficient current I1 (PTAT), can be generated by developing a voltage across another resistor, R2. For example, the voltage across resistor R2 can be generated as 1) the difference between the forward voltages of two p-n junction diodes operating at different current densities, or 2) the difference between the base-emitter voltages of two bipolar junction transistors (BJT) biased in normal active mode of operation, with the two respective base-emitter junctions having different current densities. If the implementation with the two p-n junction diodes is chosen to generate the voltage across resistor R2, the positive temperature coefficient current I1 is expressed as:I1=[VD1−VD2]/R2  (9)where VD1 and VD2 represent the forward voltages of the two diodes, respectively. If the ratio between the current densities through the two forward biased p-n junction diodes is N, equation (9) becomes:
                                                                        I                1                            =                            ⁢                                                [                                                                                    V                        t                                            ⋆                                              ln                        ⁢                                                  {                                                                                    I                              A                                                        /                            IS1                                                    }                                                                                      -                                                                  V                        t                                            ⋆                                              ln                        ⁢                                                  {                                                                                    I                              B                                                        /                            IS2                                                    }                                                                                                      ]                                /                R2                                                                                        =                            ⁢                                                (                                      k                    ⋆                                          T                      /                      q                                                        )                                ⋆                                                      [                                          ln                      ⁢                                              {                                                                              (                                                                                          I                                A                                                            /                                                              I                                B                                                                                      )                                                    ⋆                                                      (                                                          A2                              /                              A1                                                        )                                                                          }                                                              ]                                    /                  R2                                                                                        (        10        )            where IA and IB are the respective currents through the forward biased p-n junction diodes, A1 and A2 are the respective areas of the p-n junction diodes, and IS1, IS2 are the saturation currents for the respective diodes, which are proportional to their areas (IS1 is proportional to A1, IS2 is proportional to A2). In addition, Vt is the thermal voltage (k*T/q), where k=1.38*10−23 J/K and q=1.6*10−19 C and T is the absolute temperature in degrees Kelvin. If IA=IB (the current values running through the two forward biased p-n diodes are equal) and the ratio between the areas of the two p-n junction diodes is N (i.e., the ratio between A2 and A1 is N), then equation (9) becomes:I1=(k*T/q)*[ln(N)]/R2  (11)The ratio N between the areas of diodes D1, D2 is usually implemented by replicating the first p-n junction diode D1 a number of times (N) to generate the second diode D2 with N times larger area.
FIG. 3b shows in more detail the sources of error that may be associated with Current output Bandgap circuit 300, the type of Bandgap reference typically used at low power supply voltages. As shown in FIG. 3b, circuit 300 comprises a current generation circuit (310) and a current replication circuit (320). Circuit 310 generates the current IOUT—INT according to equation (5) as a weighted sum of a PTAT current and a CTAT current.
In some cases, the output of circuit 310 may be affected by errors due to power supply variation (εvcc), temperature variation (εtemp), and/or process variation (εprocess). Low power supply values preclude the use of cascoded devices in the current generation circuit due to voltage headroom limitations, thus increasing the power supply noise sensitivity of circuit 310 (and consequently, circuit 300). For example, a system application for a Current output Bandgap circuit (300) may require that the variation of IOUT relative to its average value (defined as Δ(IOUT)/IOUT when the power supply varies by 10%) be −60 dB for the range of power supply noise frequencies between DC and 100 kHz. However, this specification may be difficult to achieve at low power supply voltage values due to voltage headroom limitations.
Current replication circuit 320 generates the output current IOUT as an identical copy (k=1) or a linearly scaled version (k different than 1) of current IOUT—INT. Similar to circuit 310, the output of circuit 320 may also be affected by errors due to power supply, temperature and/or process variations, as well as current replication errors. The errors due to the current replication function are labeled in FIG. 3b as εreplica=εreplica(vcc, process, temperature). Due to the strict requirements for Bandgap reference accuracy in some applications (e.g. 1% accuracy over power supply, temperature and process variations for voltages generated by passing IOUT through resistors), the error introduced by the current replication circuit should be reduced to negligible values.
The current output IOUT of the Current output Bandgap circuit 300 may also be used to generate a reference voltage VREF, as stated above, by passing IOUT through a resistor. Thus, the Current output Bandgap may be used to implement a Voltage output Bandgap. However, reduced accuracy in the current replication stage (due, e.g., to replication errors when transferring IOUT—INT to IOUT in circuit 300 of FIG. 3b) reduces the accuracy of the reference voltage VREF. In the same manner, the relatively high sensitivity of the output current IOUT to power supply noise at low power supply values implies that the reference voltage VREF will also be highly sensitive to the power supply noise.
The problems described above for the Current output Bandgap and Voltage output Bandgap (i.e., low accuracy output current/voltage and high power supply noise sensitivity) become harder to solve as the power supply voltages for CMOS processes scale down toward the 1 V value and below. Consequently, a need exists for Current output Bandgap circuits and Voltage output Bandgap circuits capable of generating high accuracy reference currents and high accuracy reference voltages, respectively, with decreased sensitivity to power supply noise when supplied with relatively low voltage power supplies.